1. Field of the Invention
The present invention relates to a fixing apparatus for an image forming apparatus, such as a copier or a printer, and relates in particular to an excessive temperature rising prevention device, for preventing an excessive increase in temperature in a heating apparatus used for fixing.
2. Related Background Art
A conventional image forming apparatus employing an electrophotographic process will now be described.
A thermal fixing apparatus, which serves as image heating means for an image forming apparatus, fixes to a recording sheet an unfixed image (a toner image) that is formed thereon by image forming means. Well known thermal fixing apparatuses are a heat roller fixing apparatus that employs a halogen heater as a heating source and a film heating fixing apparatus that employs a ceramic face heater as a heating source.
Generally, the heat generating member of a ceramic heater is provided on the face of an insulating substrate mad e of AlN or Al2O3 and the like, and generates heat when power is supplied to it. In this case, power control means controls the supply of power based on temperatures detected by a temperature detecting device located above or adjacent to the ceramic heater. Normally, an excessive temperature rising prevention device, for preventing an excessive temperature rising (increase), is also provided in order to prevent the thermal fixing apparatus from entering an excessively high temperature state due to a power control means defect. A thermal fuse or a thermostat is frequently employed as an excessive temperature rising prevention device, for preventing an excessive temperature increase.
Generally, the thermostat comprises: a bimetal; a base member for holding the bimetal; contact points supported by the base member and can be opened and closed; and a cap, which is a thermosensitive face that is attached to the base member and covers the bimetal. The thermostat is substantially circular, having a shape similar to that of the bimetal.
When the thermostat is employed as an excessive temperature rising prevention device of a thermal fixing apparatus that uses a ceramic heater in which a heat generating member is printed on the insulating substrate, and when the thermostat is abutted against the insulating substrate of the ceramic heater, it is preferable that the width of the abutting (contacting) thermosensitive portion be less than that of the insulating substrate, so that the heat of the heater can be effectively sensed and the responsibility improved.
Therefore, when the thermostat is to be brought into contact with the insulating substrate of the ceramic heater, the thermostat having the external size corresponding to the width of the insulating substrate is used. In this case, since the contact points held by the base member must be formed in a small space, a complicated structure is employed to provide a satisfactory contact pressure. Further, in order to provide a satisfactory contact pressure while using a simpler structure, the external size of the thermostat is increased. Thus, it is difficult to thermally efficiently bring the thermostat into contact with the insulating substrate of the ceramic heater.
It is, therefore, one objective of the present invention to provide an excessive temperature rising prevention device that can certainly open and close contact point portion without losing the thermal responsibility on a temperature detecting portion, and a heating apparatus and a fixing apparatus that employ the device.
It is another objective of the present invention to provide an excessive temperature rising prevention device comprising: a temperature detecting portion for detecting temperature and an opening/closing portion for opening and closing electric contact point in accordance with the operation of the temperature detecting portion, wherein the opening/closing portion is extended long, and the length of the opening/closing portion in the longitudinal direction is greater than the width of the temperature detecting portion in the direction; and to provide a heating apparatus and a fixing apparatus employing the excessive temperature rising prevention device.
Other objectives will become obvious during the course of the following explanation given for the preferred embodiments.